Originally published in Popular Science Magazine, reproduced with permission of the magazine and the author (that would be me).

Previous    Next    Story Index    Periodic Table Home
Popular Science Logo

Pretty Penny

Turn your cheapest coins inside out using some hardware-store chemistry

Looking for something more interesting to do with that jar of pennies than just cash it in? One word: acid.

In most years before 1982, American pennies were 95 percent copper. Then the price of copper went up until you could get $100 worth of pennies at the bank, melt them down, and sell the metal for more than $100. So the government started using a core of cheap zinc with only a thin plating of copper.

The fact that pennies are made of two different metals opens up the interesting possibility of separating them. I learned how from a high-school student and family friend, Niels.

Hydrochloric acid (HCl), known in the hardware store as muriatic acid, is sold for cleaning concrete. But it will also dissolve the zinc core of a penny, leaving a foil of copper roughly a thousandth of an inch thick bearing the original image of the coin. This works because zinc is significantly more reactive than copper. In the presence of HCl it is rapidly oxidizeda^\200'converted into zinc ions that dissolve in the solution.

What about removing just the copper to leave a shiny zinc penny? You can't do that with acid, because any acid that dissolves copper will eat zinc much faster. I wasn't sure it was even possible, but my chemist friend Tryggvi came through with a method involving cyanide and persulfate. It works, but it's most definitely not something you should try at home (the process uses enough poison to wipe out everyone in a city block).

Together, these two methods let me prepare this real-life exploded viewa^\200'proving that what the U.S. Mint has joined together, an Icelandic chemist and an American teenager may put asunder.

1. File around the edge of a post-1982 penny until you see white zinc.
2. Don protective gear, and drop the penny into a small bowl of undiluted hardware-store muriatic acid.
3. Wait 15 to 60 minutes until the bubbles stop.
4. With the faucet running, pour the acid into the sink through a strainer to catch the penny. (Never pour water into acida^\200'it can boil instantly and splatter acid in your face. Always pour the acid into water.)
5. Run water through the strainer for several minutes to rinse the penny half-shells and flush out the acid.

Scan of printed version:
Periodic Table Poster   My periodic table poster is now available!

Making Cents: You can easily create a copper penny shell, or foil [inset]. The zinc-only penny in the middle, though, took a professional chemist and a lot of cyanide.

Photo Credits:
Theodore Gray/Mike Walker
Mike Walker
Mike Walker
Mike Walker